HVAC&R systems, which may include residential and commercial heat pumps, air conditioning, and refrigeration systems, employ a vapor-compression cycle (VCC) to transfer heat between a low temperature fluid and a high temperature fluid. In many VCC based systems referred to as direct-exchange systems, the “fluid” is the air in a conditioned space or an external ambient environment. In other VCC based systems, including indirect-exchange systems such as chillers, geothermal heat pumps and the like, the fluid to and from which heat is exchanged may be a liquid such as water or an anti-freeze.
VCC based systems are generally known in the art and employ a refrigerant as a medium to facilitate heat transfer. The systems are mechanically “closed” in that the refrigerant is contained within the mechanical confines of the system and there is a mechanical buffer where the heat is to be exchanged between the refrigerant and the external fluid(s). In these systems, the refrigerant circulates within the system, passing through a compressor, a condenser, and an evaporator. At the evaporator, heat is absorbed by the refrigerant from the space to be cooled in the case of an air conditioner or refrigerator, and absorbed from the external ambient or other heat source in the case of a heat pump. At the condenser, heat is rejected to the external ambient in the case of an air conditioner or refrigerator, or to the space to be conditioned in the case of a heat pump.
Most VCC based systems circulate the refrigerant through coils in the evaporator and condenser to exchange heat. In an air conditioning system, the evaporator coils absorb heat from the space to be cooled and the condenser coils reject the heat absorbed by the evaporator coils to the ambient, usually the outside air. If the air conditioning system is operating in heat pump mode, then the functions of the coils are reversed and the condenser coils absorb heat while the evaporator coils reject heat to the ambient.
Coil frosting or icing can occur when condensation on the evaporator coils (which is normal and beneficial to reduce humidity in a conditioned space) freezes, significantly reducing air flow over the coils. In an air conditioning system, ice can develop on the evaporator coils for a number of reasons, including decreased airflow across the coils due to a failed evaporator fan, low refrigerant level due to leakage, and the like. Icing can cause significant reduction in system efficiency and can result in near total loss of system cooling capacity if the system continues to run while building up more ice. Most air conditioning systems are designed such that the evaporator coil will not freeze under normal conditions. However, heat pumps and refrigerators are often designed (and freezers must be designed) such that the operating evaporator coil temperature is less than the freezing temperature of water. Frosting of the evaporator coils of these systems is expected.
Most VCC based systems in which it is expected that frosting will occur on the evaporator come equipped with a means to defrost the coils. Refrigerators and freezers, for example, usually have a defrost cycle that heats the evaporator coils for a certain period of time, typically about 30 minutes. During the defrost cycle, the compressor is disabled, an evaporator heating element is energized, and a stirring fan blows air over the evaporator coils. The heating element remains energized as long as the temperature sensed by a thermostat near or on the evaporator assembly remains below a set point temperature and above the freezing point of water. This thermostat is connected in series with the heating element such that when the set point temperature is reached, a circuit opens and current to the heating element is cut. The set point temperature is selected such that under normal conditions, the evaporator temperature is significantly above the freezing point of water, which helps ensure all frost on the evaporator is melted. Stirring fans typically blow air across the evaporator coils while defrosting to ensure the resulting liquid water is removed from the coils.
Heat pumps are particularly egregious energy wasters while defrosting. Heat pumps are equipped with “reversing valves,” which allow reversal of the flow of refrigerant through the system. In this way, a heat pump can operate as an air conditioner or a heater. In the air conditioning mode, the coil that functions as the evaporator is typically located within the conditioned space, while the coil serving the condenser function is located in the outdoor ambient. In the heating mode, refrigerant flow is reversed so the evaporator function is located outdoors, while the condenser function is located indoors. In the heating mode, the evaporator function often accumulates frost and this is anticipated in the design. Unlike refrigeration systems, heat pumps are generally not equipped with defrost heaters, but generally do follow a defrost cycle. To defrost the heat pump outdoor coil while heating, the system is “reversed” to operate in the air conditioning mode. The frosted coil located outside is then heated internally by the system operating as an air conditioner, which melts the frost. However, the conditioned space is being cooled during defrost, when it should be heated. To compensate, supplemental heating is applied, usually in the form of electric strip heaters. A typical heat pump system is both air conditioning and heating simultaneously while defrosting—a tremendous waste of energy.
Existing VCC based systems do not provide a way to determine when ice or frost has accumulated on the coils. The systems typically rely on an empirical model from manufacturers that is usually based on ambient temperature along with time of operation of the systems. For example, a refrigeration system may initiate a defrost cycle every 8 hours or after the compressor has accumulated 8 hours of run time regardless of whether frost has accumulated on the evaporator coils. Heat pumps may take into consideration the outdoor temperature in determining when to defrost, but not the actual condition of the evaporator coil. Such solutions tend to be conservative by design and hence energy wasteful, defrosting the coils well before it is absolutely necessary under most conditions to thereby ensure the equipment does not lose any heat transfer capacity.
Accordingly, what is needed is a way to more accurately detect when ice or frost may have accumulated on HVAC&R system coils and to defrost the coils based on such detection.